In modern high speed, high quality electrostatographic reproduction apparatus (copier/duplicators or printers), a latent image charge pattern is formed on a uniformly charged dielectric support member. Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the support member. The dielectric support member is then brought into contact with a receiver member, such as paper, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member and the image is fixed to the receiver member by heat and/or pressure to form a permanent reproduction thereon. Preferred support members may comprise a photoconductor or an electrographic recording member, both of which are broadly referred to herein as a primary image-forming member.
The prior art has recognized certain advantages to not providing direct contact between the receiver member and the primary image member. Thus, the use of intermediates in both single color image formation and multicolor image formation is suggested in the prior art. For example, FIG. 8 of U.S. Pat. No. 4,712,906 shows a series of single color images being formed on a primary image forming member. The single color images are transferred in registration to an intermediate roller to create a multicolor image on the surface of the roller. A multicolor image is then transferred in a single step to a receiver sheet at a position remote from the primary image forming member. This system is particularly advantageous in forming multicolor marking particles images, because the receiver sheet does not have to be attached to a roller for recirculation and can be fed along a substantially straight path. It can also be used with single color marking particles image formation for a number of other reasons including facilitating duplex and preventing contact between a primary image-forming member and a receiver sheet which may contaminate the image member with paper fibers and the like.
The use of a non-compliant intermediate transfer member in electrophotographic machines to transfer toner from an imaging member to a print media (e.g., paper) is well known. Both Rimai et al, U.S. Pat. No. 5,084,735 (1992) and Zaretsky and Gomes, U.S. Pat. No. 5,370,961 (1994) have shown that by using an intermediate transfer member (ITM) composed of a thick compliant layer with a relatively thin stiff overcoat improves the transfer efficiency of toner compared to non-compliant intermediates. Zaretsky, U.S. Pat. No. 5,187,526 (1993) points out that transfer can be improved by separately specifying the resistivity of the ITM and the second transfer roller, which forms a nip for transfer to paper. The difficulty encountered by the aforementioned techniques of utilizing intermediate transfer rollers is the limitation imposed by air breakdown (ionization) in the vicinity of the nip in which the toner is transferred from the ITM to the print media. Air breakdown degrades the transfer efficiency and image quality of toner images, especially multicolor images, by altering the quantity of charge on the toner particles. In practice, these difficulties are amplified because the rollers backing the paper are typically composed of materials that are sensitive to fluctuations in temperature and relative humidity. Furthermore, the need to reliably detack paper from the ITM is complicated and imparts further constraints on the design of the ITM that increase the cost of the system and degrades image quality. Difficulties are especially encountered when a wide range of print media are used, e.g. card stock, clay coated papers, resin coated paper, transparency, and conventional paper that has been dried by exposure to low humidity or sent through a fusing device.
In order to overcome many of these problems, there is suggested in U.S. application Ser. No. 08/900,696 the use of an insulating transfer endless web in conjunction with a compliant intermediate member. The endless web is provided to support the receiver sheet in a transfer nip with the ITM. An electrical field of a bias suited for transfer is established in the nip for electrostatically transferring a marking particle image on the ITM to a receiver sheet brought into intimate contact with the ITM in the nip. In the various embodiments described in the above-referenced application, several feature a transfer roller that is urged against the underside of the endless belt to provide pressure in the transfer nip. The transfer roller is also electrically biased to establish an electrical field for urging marking particles on the ITM to the receiver sheet in the nip. In analyzing the nature of transfer from the ITM to the receiver sheet, it appears that transfer can be improved by providing a transfer roller that is relatively small in diameter so that the wrap of the endless web on the ITM is larger than the transfer roller/ITM nip length. Additionally, it is desirable that the transfer roller apply adequate pressure to insure intimate contact of the receiver sheet with the marking particle image on the ITM.
In order to insure that the above conditions suited for successful transfer are provided, the inventors have found that when a support roller is engaged against a small diameter transfer roller, the support roller prevents the transfer roller from bending and thus minimizes distortion in the uniformity of the nip along the length of the transfer roller and provides for a more uniform nip pressure. Improved uniformity in the nip provides for more uniformity in the electrical transfer field and thereby provides for improved conditions for image transfer.